1. Field of the Invention
The present invention relates to fluid level detection systems and, more particularly, to an advantageous low-water cut-off system that operates in any of a wide range of conditions and provides precise level detection with an extraordinarily wide variety of electrically conductive fluids.
Although the term low water cut-off system is herein used, it will be understood as being applicable to systems using fluids other than water which exhibit electrical conductivity that can be sensed for the present purposes. The term low water cut-off system is used in its general sense, regardless of the fluid to be sensed.
2. Known Art
Low water cut-off systems have long been used in low-pressure and hot water boilers to cut off a burner for the boiler if there is a low water level (or other low fluid level) condition. Such low water cut-off systems may be used either for primary or secondary protection. A wide variety of such low water cut-off systems have been known or proposed, based typically either on float level detection or on electrical conductivity of a fluid whose level is to be measured.
Among the challenges in the design of low water cut-off systems are the following requirements:
The system should be preferably adjustable to provide preselected delay in its make or break operation. For example, fluid level in a boiler may fluctuate during operation, causing a possibility of possibly spurious, inaccurate or premature operation. A preselected delay may accordingly be preferred before the low water cut-off system closes control contacts (delay on make operation). Further, in the event of operation of the low water cut-off system, and/or a preselected delay may be preferred before the low water cut-off system opens control contacts (delay on break operation).
Moreover, the nature of the fluid (such as water in a boiler system) may be different from one installation to the next, being dependent, for example on impurities or additives in the fluid. Electrical conductivity of the fluid to be sensed by the low water cut-off system may be subject to a wide possible range of purity levels of the water disposed in the boiler tank.
Because low water cut-off systems of the type operating according to electrical conductivity of water or other liquid require the fluid to be conductive in order to sense its presence, high purity water, which exhibits very low electrical conductivity, can fool the device into a false low water condition. Previously, individual or special models had to be offered to the industry to handle these pure water applications.
Accordingly, it is preferred that a low water cut-off system using a probe be responsive to electrical conductivity and be capable of adjustment in probe sensitivity; however the prior art has not provided a satisfactory low water cut-off system with suitable probe and electronics arrangement in which the system is responsive to electrical conductivity. The low water cut-off system can if desired, operate under microprocessor control. It is preferred to be capable of ready, convenient adjustment in probe sensitivity over a suitable range.
It is further preferable that a low water cut-off system be capable of use over such the wide range of purity levels mentioned above. It is also preferable that a low water cut-off system be fault tolerant no matter at what water purity it is adjusted to operate.
A difficulty in achieving such performance is that the designer is confronted, on the one hand, with a need for scaling, or attenuating, the input signal provided by a signal passing through water to be monitored at a current level which is dependent upon the conductance of the water; but also, on the other hand, with a need to establish a signal comparison threshold which will assure stable operation by the monitoring circuitry. The low water cut-off system should also advantageously operate in a fail-safe mode to assure that either it will operate normally or fail safely.
In general, there is also a continuing need in the art for a low-water cut-off system that reliably prevents false or premature cut-off operation in the event of temporary changes or surges in fluid level. There is a further need in the art for a low-water cut-off system that allows selective easily-set control of both make and break times during operation, and that avoids spurious, inaccurate or premature operation of the system.
Within the art, there also is a general failure to teach a low water cut-off system which could allow for rapid and convenient adjustment of the conductivity level of the fluid with which the probe is used in order to account for the purity level of the water or other fluid in the boiler tank.
Finally, there is a need in the art for a low-water cut-off system that provides combinations of these features for use in a wide variety of low-pressure steam and hot water boilers, as well as the capability for being used in both manual and automatic reset modes.
Among the many patented low water cut-off system systems, none can be found that appears more pertinent or relevant than U.S. Pat. No. 5,111,691 to John et al. which discloses a low water cut off conductance probe and control system having a time delay feature. The probe combines temperature and electrical conductivity sensing operations. However, the time delay feature utilized in said John et al. reference is fixed such that the system cannot be adjusted by the user without changing out the particular hardware arrangement described in the reference. The time delay feature of that patent has an R-C (resistive-capacitance) charging circuit in combination with an analog timer device that turns on and off an output relay to the boiler when charged or discharged by the charging circuit. More specifically, when the probe permits current flow through water filling the tank of the boiler the charging circuit begins to cause charging of a capacitor of a timing circuit. After a fixed period of time, the capacitor voltage reaches a level to enable an analog timer device which in turn activates the boiler through an output relay. Conversely, when the water level drops below a predetermined level, the probe no longer senses the electrical charge and the charging circuit discharges the capacitor over a fixed period of time. When the charging circuit is fully de-energized the analog timer device turns off the boiler.
In contrast to the John et al. reference, the present invention provides a low water cut-off system having an adjustable time delay feature implemented by a microprocessor, rather than being part of a fixed hardware circuit arrangement as found in the John et al. reference.
In further contrast with the John et al. reference, the low water cut-off system of the present invention allows a user to simply set the preferred time delay for the system at the microprocessor in order to adjust the time delay. The John et al. reference does not teach nor suggest such an adjustable time delay system much less one which is microprocessor-implemented and readily adjustable in time delay as well as other operational parameters.
Moreover, the John et al. reference does not teach nor suggest the provision of the new low water cut-off system according to the present invention for setting or adjusting the probe conductivity to the purity level of the water or other fluid in a boiler.
Finally, prior art low water cut-off systems for steam boilers that provide for delay on make and delay on break operations are not compatible with hot water boilers that do not utilize such make or break operations; therefore, separate systems must be used for each type of boiler. The present invention solves this problem by providing a single low water cut-off system that is compatible with both steam and hot water boiler applications.